1. Related Applications
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/236,844, filed Sep. 29, 2000, entitled “FREQUENCY HOPPING DATA RADIO.”
2. Field of the Invention
The present invention relates to radio frequency transmission systems. More particularly, the present invention relates to systems and methods that utilize a received signal to provide low cost, low power, high sensitivity, radio frequency hopping.
3. Background and Related Art
Wireless or radio frequency transmission is becoming increasingly more important in today's technology. Dispensing with the need for cables and complicated wiring, wireless technology enables networks to be connected quickly and easily. Several network-based applications may find wireless technology particularly advantageous, including supervisory control and data acquisition (SCADA), remote meter reading, home automation, instrument monitoring, point-of-sale (POS) systems, wireless local area networks (WLANs), and many other applications.
One technique for radio transmission involves frequency hopping spread spectrum technology. This technique involves changing the transmission signal to a different frequency several times a second to provide a broadband distribution. A psudo-random table is used to define the hopping sequence. The broadband distribution makes the signal transmission less likely to interfere with or to be interfered with by other signals since the transmitters only occupy a potentially used frequency for a fraction of a second. For example, because many technologies use the 900 MHz ISM band (e.g., cell phones, pagers, and cordless phones), existing 900 MHz radios are susceptible to interference and may not operate at all when close to pager or cell phone towers. Spread spectrum frequency hopping helps minimize this interference.
Frequency hopping systems, however, have their disadvantages as well. For example, the signal receiver must be able to synchronize with or track the frequency “hops” so that the receiver can properly capture and demodulate the signal. In typical frequency hopping systems, both the transmitter and the receiver have a clocking/timer mechanism to achieve this tracking. This clocking mechanism is typically quite complicated and expensive to build. Furthermore, frequency hopping systems typically use a receive signal strength indicator (“RSSI”) to determine if a valid signal is present on a channel. Such indicators have the drawback that they don't reliably operate at the same sensitivity levels in which actual data receivers are capable of operating.
Radio transmission technology also involves digital modulation schemes used to modulate digital signals for radio transmission. One step in some modulation schemes entails transforming or encoding raw digital data, comprising of a series of bits in binary code, into a digital square wave signal that can be sent by a radio transmitter to a radio receiver. For example, one modulation scheme encodes raw data by transmitting a high voltage to represent a 1-bit and a low voltage for a 0-bit. The problem with this type of encoding is that when several identical bits are sent in succession the receiver cannot inherently differentiate between when one bit stops and the next bit starts. As a result, several improved encoding schemes have been developed to solve this problem.
One of the improved encoding schemes, referred to as “Manchester encoding,” transmits two voltages, one high and one low, for each bit. The “normal” type of Manchester encoding transmits a high and then low voltage for a 1-bit, and a low and then high voltage for a 0-bit. Another improved encoding scheme, referred to as the “Differential Manchester encoding,” indicates each bit by looking at the last half of the previous bit's signal. In particular, a Differential Manchester signal indicates a 1-bit by transmitting the first half of the signal at a voltage equal to the last half of the previous bit's signal. The first half of a 0-bit signal is transmitted at the voltage level opposite to the last half of the previous bit's signal. As with a normal Manchester signal, the Differential Manchester represents each bit by two voltages. In other words, there is a transition between signal levels in the middle of the portion of the signal that represents each bit. Thus, because each bit representation in a Manchester signal has a transition in the middle, the radio receiver is able to synchronize more easily with the radio transmitter. Therefore, the middle transition points serve as inherent “markers” that allow the radio receiver to differentiate between when one bit ends and another bit starts. Typically, a Differential Manchester signal is determined by looking for the frequency component created by the periodic transitions in the data. This frequency is used to enhance the recovery of the data by locking the recovery circuitry to the exact frequency of the transmitted data.
Thus, while several types of frequency hopping spread spectrum systems currently exist, there is nevertheless a need for frequency hopping spread spectrum technology that is inexpensive, is reliable, and maintains high sensitivity.